The escalating requirements for high density and performance associated with ultra large scale integration semiconductor wiring require responsive changes in interconnection technology, which is considered one of the most demanding aspects of ultra large scale integration technology. Such escalating requirements have been found difficult to satisfy in terms of providing a low RC (resistance capacitance) interconnect pattern, particularly wherein submicron vias, contacts and trenches have high aspect ratios due to miniaturization.
Conventional semiconductor devices comprise a semiconductor substrate, typically doped monocrystalline silicon, and a plurality of sequentially formed dielectric interlayers and conductive patterns. An integrated circuit is formed containing a plurality of conductive patterns comprising conductive lines separated by interwiring spacings, and a plurality of interconnect lines, such as bus lines, bit lines, word lines and logic interconnect lines. Typically, the conductive patterns on different layers, i.e., upper and lower layers, are electrically connected by a conductive plug filling a via hole, while a conductive plug filling a contact hole establishes electrical contact with an active region on a semiconductor substrate, such as a source/drain region. Conductive lines formed in trench openings typically extend substantially horizontal with respect to the semiconductor substrate. Semiconductor "chips" comprising five or more levels of metallization are becoming more prevalent as device geometries shrink to submicron levels.
A conductive plug filling a via hole is typically formed by depositing a dielectric interlayer on a conductive layer comprising at least one conductive pattern, forming an opening through the dielectric interlayer by conventional photolithographic and etching techniques, and filling the opening with a conductive material, such as tungsten (W). Excess conductive material on the surface of the dielectric interlayer is typically removed by chemical-mechanical polishing (CMP) One such method is known as damascene and basically involves the formation of an opening which is filled in with a metal. Dual damascene techniques involve the formation of an opening comprising a lower contact or via hole section in communication with an upper trench section, which opening is filled with a conductive material, typically a metal, to simultaneously form a conductive plug in electrical contact with a conductive line. In copending application Ser. No. 08/320,516 filed on Oct. 11, 1994, prior art single and dual damascene techniques are disclosed, in addition to several improved dual damascene techniques simultaneously forming a conductive line in electrical contact with a conductive plug for greater accuracy in forming fine line patterns with minimal interwiring spacings.
High performance microprocessor applications require rapid speed of semiconductor circuitry. The control speed of semiconductor circuitry varies inversely with the resistance and capacitance of the interconnection pattern. As integrated circuits become more complex and feature sizes and spacings become smaller, the integrated circuit speed becomes less dependent upon the transistor itself and more dependent upon the interconnection pattern. Thus, the interconnection pattern limits the speed of the integrated circuit.
If the interconnection node is routed over a considerable distance, e.g., hundreds of microns or more, as in submicron technologies, the interconnection capacitance limits the circuit node capacitance loading and, hence, the circuit speed. As integration density increases and feature size decreases in accordance with submicron design rules, the rejection rate due to integrated circuit speed delays approaches and even exceeds 20%.
One way to increase the control speed of semiconductor circuitry is to reduce the resistance of a conductive pattern. Conventional metallization patterns are typically formed by depositing a layer of conductive material, notably aluminum (Al) or an alloy thereof, and etching, or by damascene techniques wherein trenches are formed in dielectric layers and filled with a conductive material. Excess conductive material on the surface of the dielectric layer is then removed by CMP. Al is conventionally employed because it is relatively inexpensive, exhibits low resistivity and is relatively easy to etch. However, as the size of openings for vias/contacts and trenches is scaled down to the sub-micron range, step coverage problems have arisen involving the use of Al which has decreased the reliability of interconnections formed between different wiring layers. Such poor step coverage results in high current density and enhanced electromigration. Moreover, low dielectric constant polyamide materials, when employed as dielectric interlayers, create moisture/bias reliability problems when in contact with Al.
One approach to improved interconnection paths in vias comprises the use of completely filled plugs of a metal, such as W. Accordingly, many current semiconductor devices utilizing VLSI (very large scale integration) technology employ Al for a wiring metal and W plugs for interconnections at different levels. However, the use W is attendant with several disadvantages. For example, most W processes are complex and expensive. Moreover, W has a high resistivity. The Joule heating may enhance electromigration of adjacent Al wiring. Furthermore, W plugs are susceptible to void formation and the interface with the wiring layer usually results in high contact resistance.
Another attempted solution for the Al plug interconnect problem comprises the use of chemical vapor deposition (CVD) or physical vapor deposition (PVD) at elevated temperatures for Al deposition. The use of CVD for depositing Al has proven expensive, while hot PVD Al deposition requires very high process temperatures incompatible with manufacturing integrated circuitry.
Copper (Cu) and Cu alloys have received considerable attention as a candidate for replacing Al in VLSI interconnect metallizations. Cu exhibits superior electromigration properties and has a lower resistivity than Al. In addition, Cu has improved electrical properties vis-a-vis W, making Cu a desirable metal for use as a conductive plug as well as conductive wiring.
Electroless plating and electroplating of Cu and Cu alloys offer the prospect of low cost, high throughput, high quality plated films and efficient via, contact and trench filling capabilities. Electroless plating generally involves the controlled autocatalytic deposition of a continuous film on the catalytic surface by the interaction in solution of a metal salt and a chemical reducing agent. Electroplating comprises the electro deposition of an adherent metallic coating on an electrode employing externally supplied electrons to reduce metal ions in the plating solution. A seed layer is required to catalyze electroless deposition or to carry electrical current for electroplating. For electroplating, the seed layer must be continuous. For electroless plating, very thin catalytic layers, e.g., less than 100 .ANG., can be employed in the form of islets of catalytic metal.
There are disadvantages attendant upon the use of Cu or Cu alloys. For example, Cu is easily oxidized and vulnerable to corrosion. Unlike Al, Cu does not form a self-passivating oxide on its surface. Accordingly, corrosion of Cu is an important issue which requires resolution before Cu can be effectively utilized in many semiconductor device applications. Moreover, Cu readily diffuses through silicon dioxide, the typical dielectric interlayer material employed in the manufacture of semiconductor devices, into silicon elements and adversely affects device performance.
One approach to forming Cu plugs and wiring comprises the use of damascene structures employing CMP, as in Chow et al., U.S. Pat. No. 4,789,648. However, due to Cu diffusion through dielectric interlayer materials, such as silicon dioxide, Cu interconnect structures must be encapsulated by a diffusion barrier layer. Typical diffusion barrier metals include tantalum (Ta) , tantalum nitride (TaN), titanium nitride (TiN), titanium-tungsten (TiW), and silicon nitride (Si.sub.3 N.sub.4) for encapsulating Cu. The use of such barrier materials to encapsulate Cu is not limited to the interface between Cu and the dielectric interlayer, but includes interfaces with other metals as well.
In copending application Ser. No. 08/857,129, filed May 15, 1997, now U.S. Pat. No. 5,969,422, issued Oct. 19, 1999 a method of electroless plating or electroplating copper or a copper alloy to fill high aspect ratio openings is disclosed, wherein a seed layer comprising an alloy of a refractory metal and one or more additive metals is initially deposited.
Copending application Ser. No. 08/587,264, filed Jan. 16, 1996, discloses a method of electrolessly depositing Cu in an interconnect structure, which method comprises initially depositing a barrier layer in an opening, depositing a catalytic seed layer, preferably of Cu, on the barrier layer, and then depositing a protective layer the catalytic layer encapsulating and protecting the catalytic layer from oxidation. The preferred protective material is Al which forms an Al--Cu alloy at the interface of the catalytic and protective layers, thereby encapsulating the underlying Cu. Subsequently, Cu is electrolessly deposited from an electroless deposition solution which dissolves the overlying protective alloy layer to expose the underlying catalytic Cu layer.
Al has been suggested as a doping material to improve the oxidation and corrosion resistance of Cu and act as a diffusion barrier by forming a thin protective layer of aluminum oxide on the surface of a Cu layer upon annealing. P. J. Ding et al., "Effects of the Addition of Small Amounts of Al to Copper: Corrosion, Resistivity, Adhesion, Morphology, and Diffusion". J. Appl. Phys., 75(7) 1994, 3627-3631. Magnesium (Mg) has also been suggested as a doping material to improve the oxidation resistance of Cu.
There exists a need for semiconductor technology enabling the formation of Cu or Cu alloy metallization in interconnection patterns with improved corrosion resistance while substantially reducing or eliminating Cu diffusion.